1. Field of the Invention
The present invention generally relates to electrical circuit components, and more specifically, to the design, operation and method of manufacture of an efficient inductor and related systems thereof.
2. Prior Art
Inductors have been extensively utilized in electrical circuits for many years dating back to the late 1800s. Inductors are utilized in just about every electrical circuit and they play a vital role in the operation of numerous electronic devices from modern televisions to satellite communication systems. There are two common types of prior art inductors, the first type are wire wound inductors, and the second type are ceramic based inductors. Wire wound inductors have historically been constructed of a metal coil that is wrapped around a core of air, paramagnetic, or ferromagnetic material. Ceramic-based inductors are typically multilayer, film or wire-wound technologies, each having features that provide characteristics suitable for various applications.
In an inductor, electric current travels through the metallic coil generating a magnetic flux that is proportional to the amount of electric current. A change in electrical current elicits a corresponding magnetic flux proportional to the amount of current, which in turn, generates an electromotive force (EMF), measured in volts, that opposes the change in current. Inductance is a measure of the amount of EMF generated per unit change in current. For example, an inductor with an inductance of 5 henries produces an EMF of 5 volts when the current through the inductor changes at a rate of 5 amperes per second.
A pure or “ideal inductor” is an inductor that is one hundred percent efficient. Such an ideal inductor does not dissipate or radiate energy. However, inductors utilized in electrical circuits are not theoretical ideal inductors, but rather, are “real inductors”, in that they have internal losses that dissipate or radiate energy and contribute to the overall inefficiency of the inductor. Energy loss within an inductor is generally due to internal electrical resistance which is generally the result of the traditional structure and design of an inductor, for example, wherein a coil is wrapped around a core of air or some material or wherein a coil structure is associated with a ceramic substrate.
Specifically, the electrical resistance within an inductor is generally caused by the cumulative effects of the electrical resistance of the coil structure that is either a wire wrapped around a core material or a trace, film or mounted wire on a ceramic substrate. This internal loss becomes more pronounced as the operating frequency is increased. At high frequencies, particularly at radiofrequencies (RF) and greater, inductors of the prior art, typically have higher electrical resistance and other losses. In addition to causing power loss, in inductance circuits this can reduce the quality factor (Q factor) of the inductor and the electrical circuit, broadening the bandwidth. In prior art ceramic based inductors, for example, Q factor values at of about 5 to about 30 are generally achieved at a given frequency. Prior art wire wound inductors with either air or ferrite cores have Q values on the order of 50 to 100. Furthermore, the Q values of these prior art inductors significantly degrade with increasing operating frequency.
The multi-layer, multi-turn inductor of the present invention performs at greater efficiencies in a similar volume and at similar efficiencies in a substantially smaller volume. In particular, the inductor of the present invention performs at greater efficiencies, particularly at RF frequencies and greater. In operation, the multi-layer, multi-turn inductor of the present invention generally has a Q factor that is about 20 to 30 percent greater than the inductor designs of the prior art.
The relatively low quality factor of these inductors is mainly due to higher resistive losses caused by a phenomenon known as the “skin effect.” Generally, skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor such that the current density is more predominant near the surface of the conductor with the remaining conductor body ‘unused’ relative to electrical current flow. The remaining conductor body is ‘unused’ relative to electrical current flow because the current density typically decays with distance therewithin away from the surface of the conductor. The electric current flows mostly near the surface, and is referred to as the “skin” of the conductor. The depth at which the current decays to about 37% of the magnitude than at the surface is called the “skin depth.” The “skin depth” then defines the electrical current cross-sectional area that is carries most of the current (is active) in the conducting wire of an inductor, whether the inductor wire is wire that is wound around a core material, or a wire that is a trace, a film or a mount on a ceramic substrate.
In inductors, particularly those operating in the RF frequency range and above, the skin effect phenomenon generally causes energy loss as current flows through the wire of the inductor and circuit. Higher resistive loss at high frequencies is a problem faced by most electronic devices or appliances. Skin effect becomes more prevalent when operating frequency increases. With higher frequencies, current that normally flows through the entire cross section of the wire comprising the inductor becomes restricted to its surface. As a result, the effective resistance of the wire is similar to that of a thinner wire rather than of the actual diameter through which the current could be distributed. A wire exhibiting tolerable resistance for efficient performance at low frequency transitions into a wire of unacceptable resistance at high frequency. The transition from tolerable to unacceptable resistance translates into inefficient lower quality factor values of the inductor and overall electrical circuit. Additionally, current inductor designs do not resolve these inefficiencies, and, in some cases, exacerbate the inefficiencies of the electrical circuit, particularly at high RF frequencies. Although not exhaustive, typical applications limited by current inductor technology include, for example, radio frequency identification (RFID), battery charging and recharging, telemetry, sensing, communication, asset tracking, patient monitoring, data entry and/or retrieval, induction heating, electromagnetic field generation, RF matching, RF chokes, RF MEMs, electronic switching, interference filtering, oscillators, amplifiers, induction heating, microwave circuits, magnetic resonance imaging, and the like. Further, these inductor fabrication techniques are relatively complex and are cost prohibitive.
In RFID applications, such as supply chain management, product authenticity, and asset tracking, there is a need to increase read range, increase read rates, improve system reliability and improve system accuracy. At high frequency for example, read range is at most three feet which is generally insufficient for pallet tracking. Ultra high frequency readers enable greater read distances of eight to ten feet, however, they introduce other performance issues like signals that are reflected by metal or are absorbed by water, or display unreadable, null spots in read fields. Increased read range requires concentrated power to facilitate reflecting back the signal for better performance, hence, a more efficient structure could help solve these issues.
In applications requiring efficient low loss coils which need to maintain inductance under harsh conditions, conventional wire-based inductors could be deformed. It is well known that any deformation of the wire cross-section will lead to a change in inductance and possibly resistance, which in turn will change the resonance frequency of the inductor and consequently may increase overall system resistance and degrade system performance. Improved methods of manufacturing these types of structures that reduce the potential for compromising deformation could eliminate this problem. The present teachings include methods of manufacture that include both rigid structure designs and flexible structure designs.
Litz wires were developed, in part, in an attempt to address the issues discussed above. However, Litz wires are generally insufficient for use in high frequency applications, and are therefore generally not useful in applications having operating frequencies above about 3 MHz. Furthermore, inductors constructed with Litz wire tend to deform under physical stresses and deteriorate when exposed to harsh environmental conditions. A Litz wire is a wire consisting of a number of individually insulated magnet wires twisted or braided into a uniform pattern, so that each wire strand tends to take all possible positions in the cross-section of the entire conductor. This multi-strand configuration or Litz construction is designed to minimize the power losses exhibited in solid conductors due to “skin effect” and “proximity effect”. Litz wire constructions attempt to counteract this effect by increasing the amount of surface area without significantly increasing the size of the conductor. However, even properly constructed Litz wires exhibit some skin effect due to the limitations of stranding. Wires intended for higher frequency ranges generally require more strands of a finer gauge size than Litz wires of equal cross-sectional area, but these higher frequency wires are composed of fewer and larger strands. Further, the highest frequency at which providers of Litz wires offer configurations capable of improving efficiencies is about 3 MHz. There is currently no solution for applications with operating frequencies beyond this 3 MHz maximum frequency limit. Additionally, there is currently no solution that improves efficiency in a given size or provides similar efficiency in a smaller size.
Hence a need exists for an improved high efficiency design and method of manufacture that reduces the intrinsic resistive losses of the inductor structure, and in particular reduces intrinsic resistive losses of the inductor at high frequencies to achieve high quality factors.